Dr. Alon Davidy, Heat Transfer Researcher
In an oil refinery, hydrogen sulfide (H2S) can be generated from various sources during the processing of crude oil. Some of the major sources of H2S in an oil refinery include:
Crude oil: Hydrogen sulfide can naturally occur in crude oil deposits, and its concentration varies depending on the oil source. As crude oil is processed in the refinery, H2S can be released from the oil stream.
Hydro-treating units: Hydro-treatment is a refining process that involves the use of hydrogen to remove impurities, such as sulfur compounds, from various refinery products, such as gasoline, diesel, and jet fuel. The removal of sulfur compounds results in the release of hydrogen sulfide gas.
Hydrocracking units: Hydrocracking is a process that converts heavy hydrocarbons into lighter products using hydrogen as a catalyst. During this process, sulfur-containing compounds are broken down, releasing H2S.
Coking units: Coking units convert residual heavy oils into lighter products and petroleum coke. Sulfur compounds in the residual oils can decompose during coking, leading to the release of H2S.
Sour water strippers: Sour water is generated in various refinery units that contain hydrogen sulfide and ammonia. Sour water strippers are used to remove H2S from this water stream, and the stripped gas contains H2S.
Delayed coker units (DCU): Delayed coking is a process that converts residual heavy oils into petroleum coke. As in coking units, sulfur compounds in the residual oils can decompose, generating H2S.
Desulfurization units: Refineries may have desulfurization units that remove sulfur compounds from specific refinery streams. During this process, H2S can be produced as a byproduct.
In this research, a hydrogen sulfide extraction system based on ionic liquids was calculated. This system is shown in Figure 1 [1].
![Schematics of the hydrogen sulfide extraction system [1].](https://lh3.googleusercontent.com/2Haj-jhpt8cXqnoC2tKSXElestEOANRe2xAXyahg103CaiaXHd8Tlzvde2N6DIPKP9O_ZBs30EsadOIFro6zjrn7ok0pANi3JMFRfzZ_oqm1Cm_XmTTDkcnDDzuPxXVPcjwf5KTTqgtEhn9UUWL3)
Figure 1: Schematics of the hydrogen sulfide extraction system [1].
The COMSOL finite element code solved the continuity, turbulent fluid flow (by applying a k-ε model), and mass transfer equations. This work is the first coupled CFD simulation of a H2S extraction system based on an ionic liquid system and graphene oxide (GO) membrane. The proposed hydrogen sulfide extraction system contains a tube, membrane and shell. 1-ethyl-3-methylimidazolium (emim) with bis-(trifluoromethyl) sulfonylimide (NTf2) anion based ionic liquid has been selected due to their high H2S diffusion coefficient. A functionalized graphene oxide (GO) membrane was employed in this design. This membrane offers several advantages such as:
(a) The graphene oxide membrane has exceptionally high permeability to various gasses and liquids.
(b) It is chemically stable and resistant to many corrosive substances. It is suitable for use in harsh environments.
(c) It can be engineered to be selective for specific molecules or ions.
It was determined that the H2S is absorbed almost completely by the 1-ethyl-3-methylimidazolium (emim)-based ionic liquids with bis-(trifluoromethyl) sulfonylimide (NTf2) anion after a time period of 30,000 s. This time period is a function of the diffusion coefficients of the H2S in the membrane and the ionic liquid. Figure 2 provides a 3D plot of the hydrogen sulfide concentration profile at t = 3000 s inside the hydrogen sulfide extraction system [1].
Figure 2: Three-dimensional plot of the hydrogen sulfide concentration profile.
Two CFD simulations with different meshes (3708 and 14,832 elements) have been carried out using the COMSOL multiphysics finite element code. It was found that the H2S concentration profiles obtained from these two CFD simulations are almost the same. The CFD results have been validated against the analytical results. A closed-form solution was obtained for a constant surface concentration for a semi-infinite medium (the analytical solution is valid for a very short time). The analytical solution for this case could be obtained by recognizing the existence of a similarity variable, through which the diffusion equation may be transformed from a partial differential equation involving two independent variables (x and t) to an ordinary differential equation expressed in terms of the single similarity variable. It was found that the analytical results are similar to the numerical results. This extraction system has several advantages:
(a) Hydrogen sulfide (H2S) is considered toxic. This system reduces hydrogen sulfide pollution.
(b) It prevents stress corrosion cracking (SCC) to refine the piping system. H2S SCC typically occurs when a metal is under tensile stress, which is stress that tends to stretch or elongate the material. The combination of tensile stress and exposure to H2S can initiate and accelerate the cracking process. This phenomenon begins with the absorption of hydrogen atoms into the metal lattice. This can lead to the formation of hydrogen gas at specific locations, creating internal pressure that causes cracks to initiate and grow in the material. These cracks can ultimately lead to catastrophic failure of the component or structure if left unchecked. The severity of SCC is influenced by environmental factors, such as the concentration of H2S, temperature, pressure, and the presence of other corrosive substances.
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References:
[1] Davidy, A. CFD Simulation of Hydrogen Sulfide (H2S) Desulfurization Using Ionic Liquids and Graphene Oxide Membrane. Fuels 2023, 4, 363-375. https://doi.org/10.3390/fuels4030023
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